1. Field of the Invention
The present invention relates to a molded article having excellent crack resistance, more specifically to a molded article having excellent crack resistance which is integrally formed by molding a thermoplastic saturated norbornene-based polymer over an insertion part having a linear expansion coefficient of not more than 2.5.times.10.sup.-5 /.degree.C.
2. Related Art
It is known that a thermoplastic saturated norbornene-based polymer is widely usable due to its excellent transparency, heat resistance, humidity resistance, chemical resistance, etc., and also useful particularly as an optical plastic molding material for an optical disk substrate, etc., due to smallness of its birefringence.
However, a problem is that a molded article integrally formed by molding a thermoplastic saturated norbornene-based polymer over an insertion part having a smaller linear expansion coefficient is liable to crack due to thermal changes, etc., since the linear expansion coefficient of the polymer and that of the insertion part are different to a large extent.
In an optical magnetic disk, it is general practice, in order to fix said disk to a drive apparatus with a magnet, to employ a method in which a hub integrally formed by molding a resin with a metal plate insert is bonded to an optical disk substrate. As a method for bonding a hub to an optical disk substrate, there are known an ultrasonic bonding method and a method using an ultraviolet-curable adhesive, a thermosetting adhesive, a hot-melt adhesive, or the like. Of these methods, an ultrasonic bonding method is generally used since the bonding time is short and its workability is excellent. In the case of an ultrasonic bonding, however, it is difficult to bond polymers which greatly differ in chemical composition. Therefore, a hub formed of the same polymer as that of an optical disk substrate is used. For example, a combination of a substrate formed of polycarbonate with a hub formed of polycarbonate or a combination of an optical disk substrate formed of a thermoplastic saturated norbornene-based polymer with a hub formed of a thermoplastic saturated norbornene-based polymer is suitable for ultrasonic bonding. However, a hub integrally formed by molding a thermoplastic saturated norbornene-based polymer with a preferred metal such as stainless steel, etc., insert has problems in that it is liable to crack as described above and is unreliable when used under an environment having a high temperature and a high humidity. For this reason, there is employed a method in which a hub formed of polycarbonate or poly-4-methyl-1-pentene is bonded to an optical disk substrate formed of a thermoplastic saturated norbornene-based polymer with an adhesive. However, bonding workability is not necessarily good, nor is durability of the bonding strength sufficient.